Spastic muscle behavior results from neuromuscular injury or illness of the central nervous system and limits motor control and functional performance thereby contributing to disability. Unfortunately, treatment outcomes are limited because spasticity is poorly quantified and the relation between spasticity and dysfunction is poorly understood. We hypothesize 1) existing clinical spasticity assays, specifically the Ashworth test which is the primary indicator for treatment, qualitatively describes only a sub-set of the multiple dimensions of spasticity, and 2) functional constraints can be predicted from quantitatively measures of spasticity. Spasticity of individual patients with cerebral palsy will be quantitatively measures of spasticity. Spasticity of individual patients with cerebral palsy will be quantitatively measured in terms of the threshold joint angle at which spastic activation is initiated, the velocity- dependence of this threshold angle, and the post-threshold response severity. These three dimensions of spasticity will be computed from resistance force and myoelectric activity during passive isokinetic knee flexion and extension. Results will be compared with each patient's Ashworth score of the knee to determine which dimensions of spasticity the Ashworth test records. Kinematics of walking (knee joint angle and angular velocity) will be recorded to illustrate the function in these patients is achieved predominantly using sub-threshold motion patterns. Gross Motor Function Measures (GMFM) scores will be compared to the magnitude of the subthreshold region of motion demonstrating that functional limitation is related to the severity of the kinematic restrictions imposed by the spastic activation threshold. Results will provide a quantifiable measure of spasticity and improve our understanding of existing clinical assays and functional limitations associated with spasticity. These will contribute to improve clinical outcomes by developing predictive tools for clinical assessment, patient characterization and treatment evaluation. These efforts will also establish the preliminary components of a multi-disciplinary center of excellence for the clinical/biomechanical quantification and characterization of spasticity.